Enhancing the ttH signal through top-quark spin polarization effects at the LHC

We compare the impact of top-quark spin polarization effects in Higgs boson production in association with top-quark pairs and in corresponding backgrounds at the LHC. Because of the spin-zero nature of the Higgs boson, one expects, in the chiral limit for the top quarks, a substantial complementarity in tt ̄ spin correlations for a Higgs decaying into fermions/gauge-bosons and tt ̄ spin correlations for the corresponding irreducible tt ̄ff ̄/VV backgrounds. Although top mass effects in tt ̄H production are in general dominant, and seriously spoil the chiral-limit expectations, one can find observables that capture the tt ̄ angular spin correlations and can help in separating the signal from irreducible backgrounds. In particular, we show that, for both H→bb ̄ and H→γγ, taking into account tt ̄ spin correlations in tt ̄H production and irreducible backgrounds could appreciably improve the LHC sensitivity to the tt ̄H channel.